1. Field of Invention
The invention relates to a method for producing dimensionally accurate metal foam from foamable, powder-metallurgic semi-finished metal products having a melting point >200° C., as well as to devices for carrying this out.
2. Description of Related Art
Production of foam from suitable foamable material for plastics, natural substances, glasses and also metal-containing materials, is known.
Methods for powder-metallurgic metal foam production in molds having low expansion coefficient are known from German Patent Application DE 199 54 755 A1. There, AISi12 alloy is foamed in a powder metallurgical manner; however, the information given there is only suited for this material, as continuously material-dependent magnitudes are mentioned. This also holds good for the necessary 5-25 nm thick protection layer of the quartz glass mold by an Al2O3-coating of the quartz glass, as well as for the applied cover layer which is necessary on account of the reactivity of the foaming AISi12. There, also through a thick-walled mold with layer thicknesses >5 mm and an applied protection layer is coupled radiation in mid-infrared, whereby the infrared emitter is geometrically arranged in such a way, that heat sinks occur in the powder compact. This known method can only work with powder compacts which are applied on cover layers and there occur problems with nonuniform heating of the mold, which results in nonuniform foam samples and foams which are not dimensionally accurate which, particularly in the case of larger foam parts, leads to instability of the foam and hence to cracks, weak points etc.
So far, it has been extremely difficult to produce metal foam parts which are dimensionally accurate of satisfactory quality. It is a problem to achieve a uniform pore distribution in larger components, e.g., large-surfaced ones like metal foam plates with a base area of 0.5 m2 and more. Such metal foam parts produced according to the known foaming methods often have regions in which the pores are collapsed, and as a result, large hollow spaces are present which weaken the stability of the component. In case of parts with nonuniform thickness or such ones with regions of higher density, which occurs by inserting more semi-finished products at pre-determined points, particularly, very often defects occur. This is especially due to the fact that traditional molds of metal have a high linear expansion coefficient and a high heat capacity. The expansion coefficient leads to the situation, that great dimensional changes take place on cooling, which negatively influence the dimension-precision and the cooling behavior of the metal foam. Known molds or casting molds require a lot of energy for heating, due to which the cooling takes a long time and results in long cycle periods in production. The cooling can also lead to material problems in metal foam, in case composites are supposed to be foamed and too long dwelling in a fluid condition leads to undesirable reactions or dissolutions, like de-mixing phenomena. A further problem is that, in the known foam processes in furnaces, an uncontrolled heat distribution in the casting mold leads to uncontrolled foaming of the foamable material, and hence, one does not get a satisfactory pore distribution.
In other known methods, the semi-finished product is heated up in metal casting molds in a furnace to a temperature which lies clearly above the melting temperature of matrix metal of the semi-finished product. In order to achieve an adequate productivity of the process and above all good quality of the metal foam, the heating also takes place very rapidly, i.e., within a few minutes. On the other hand, a very specific heating of the foamable material is very necessary, as otherwise, individual regions of the semi-finished product do not get foamed, whereas other regions get over heated and the foam cells there collapse. Therefore, the casting mold must be heated in a very short time—e.g., with the least possible temperature differences for plane metal foam of uniform thickness—, which is particularly difficult for larger molds or casting molds and metal foam parts. A big problem in this case is the large heat capacities of known casting molds, which cannot be easily cooled rapidly and, on account of the high heat conducting capacity of the metal, do not allow locally differentiated heating.
The known method of foaming in metal molds in a furnace was disadvantageous because it was difficult to control, had to be often interrupted and, one could not run the process continuously. Finally, the energy costs were also quite high.